1. Field of the Invention
The present invention relates to an electronic device with external terminals using a high temperature lead-free solder and a method of production of the same.
2. Description of the Related Art
In general, an electronic device such as a composite multilayer capacitor is provided with external terminals bonded to terminal electrodes for electrical connection with a circuit board etc. As the method for bonding the external terminals of such an electronic device to a circuit board etc., methods using soldering are frequently used. Further, since the external terminals of the electronic device will be mounted on the circuit board by solder, they have solder heat resistance and solder wettability.
The terminal electrodes of an electronic device have underlayer electrodes comprised of copper etc. formed by coating and baking a paste on the material of the body of the electronic device, nickel plating for giving solder heat resistance, and tin plating for giving solder wettability. These plating layers and external terminals are bonded by a high temperature solder.
The external terminals and circuit board are also connected using a high temperature solder. This high temperature solder is required not to melt or be difficult to melt at a high temperature (that is, have reflow resistance) and to be able to retain mechanical strength at a high temperature. Here, a xe2x80x9chigh temperature solderxe2x80x9d means a solder having a solidus temperature of at least 183xc2x0 C.
In general, among solders comprised of various types of metal compositions, in the Pbxe2x80x94Sn system, a high temperature solder having 95 wt % of Pb and 5 wt % of Sn (solidus temperature 307xc2x0 C. and liquid temperature 327xc2x0 C.) or a high temperature solder having 90 wt % of Pb and 10 wt % of Sn (solidus temperature 270xc2x0 C. and liquid temperature 301xc2x0 C.) is being used. Further, in the Pbxe2x80x94Ag system, a high temperature solder having 97.5 wt % of Pb and 2.5 wt % of Ag (solidus temperature 304xc2x0 C. and liquid temperature 304xc2x0 C.) is being used. Further, in the Pbxe2x80x94Agxe2x80x94Sn system, a high temperature solder having 97.5 wt % of Pb, 1.5 wt % of Ag, and 1 wt % of Sn (solidus temperature 309xc2x0 C. and liquid temperature 309xc2x0 C.) is being used.
The above high temperature solders include at least 90 wt % of Pb to reduce the cost or to obtain reflow resistance.
Further, for example, inside a coil, transformer, etc. of an electronic device, an insulated conductor is wound around a magnetic material or other support. When bonding the end of such a winding with the device etc., soldering is generally used. At the time of this soldering, unless the coating portion comprised of polyurethane etc. of the insulated conductor is destroyed by the heat of the solder etc., soldering is not possible. Therefore, as the temperature of soldering, one inside a temperature range of 380xc2x0 C. to 420xc2x0 C. is generally employed. For this reason as well, the above high temperature solders contain at least 90 wt % of Pb.
Note that at the present time, to bond a circuit board etc. and an electronic device etc., in the Pbxe2x80x94Sn system, a solder having 37 wt % of Pb and 63 wt % of Sn (solidus temperature 183xc2x0 C. and liquid temperature 183xc2x0 C.) and in the Pbxe2x80x94Agxe2x80x94Sn system, a solder having 36 wt % of Pb, 2 wt % of Ag, and 62 wt % of Sn (solidus temperature 179xc2x0 C. and liquid temperature 190xc2x0 C.) are being used. With these solders, generally soldering is performed at a reflow temperature of a temperature range of 220xc2x0 C. to 240xc2x0 C.
Note that if the solder used for internal bonding of an electronic device etc. mounted on a circuit board melts at the time of soldering, the molten solder flows out and the flowing solder becomes spherical and for example bridges circuits on a circuit board with circuits arranged at a high density and fine pitch. To avoid this, it is necessary to use a solder which does not melt or is difficult to melt (that is, has reflow resistance) even at the above reflow temperature. Therefore, in general, it is desirable to use a high temperature solder having a solidus temperature of at least 240xc2x0 C. or more even for the solder used for internal bonding of an electronic device etc.
As explained above, Pb is a metal essential for solder. Solder containing Pb is being used as the most effective solder after a long time in the process of bonding in electronic apparatuses. Its reliability has also been established. However, due to dumping etc. of electronic products using such solder in the natural world, the Pb contained in the solder gradually leaches out and is liable to lead to Pb pollution of the ground water.
Therefore, instead of the above Pb-containing Pbxe2x80x94Sn eutectic solder or close to eutectic solder, demand is rising for the development of Pb-free solder (lead-free solder). As such lead-free solder, Snxe2x80x94Ag, Snxe2x80x94Zn, and Snxe2x80x94Bi solders are promising, but the liquid temperatures of these solders are 10xc2x0 C. to 20xc2x0 C. higher than the liquid temperatures of the solders currently being used in connection of circuit boards etc. and electronic devices.
The general reflow temperature at the time of soldering is expected to be in the range from 230xc2x0 C. to 260xc2x0 C. Accordingly, it becomes necessary to use high temperature lead-free solder which will not melt or will be difficult to melt (that is, has reflow resistance) even at such a reflow temperature.
Further, solders having performances equal to those of a Pb-system solder in the point of the tensile strength, load resistance, and other mechanical strength properties of the soldered parts between the terminal electrodes and external electrodes of the electronic device after soldering are also being demanded.
When generally soldering composite multilayer capacitors, circuit modules, and other electronic devices, high temperature solder is often used, but for example a medium-high temperature lead-free solder having a solidus temperature of at least 200xc2x0 C. sufficient in bond strength etc. has not been obtained.
That is, soldering of a conventional electronic device and external terminals is bonding by a melting reaction at the outside (electronic device side) of the nickel plating formed for the solder heat resistance of the external terminals. Therefore, the bond strength between the electronic device and external terminals was not sufficient and there were problems in mounting.
A first object of the present invention is to provide an electronic device with external terminals superior in the tensile strength, load resistance, and other mechanical strength properties of the soldered parts, superior in reflow resistance as well, and free from concern over environmental pollution, and a method of production of the same.
A second object of the present invention is to provide an electronic device with external terminals having a large tensile strength, that is, bond strength, between the terminal electrodes and external terminals, superior in reflow, and not containing lead and therefore completely environmentally friendly.
To achieve the above objects, according to a first aspect of the present invention, there is provided an electronic device with external terminals having terminal electrodes and external terminals of the electronic device body electrically bonded through solder layers, wherein said solder layers are comprised of an Snxe2x80x94Sb high temperature lead-free solder, the ratio of Sn and Sb in the solder layers being, by weight percent, within a range of Sn/Sb=70/30 to 90/10, and said solder layers and terminal electrodes are formed between them with diffusion layers formed by diffusion of the conductive ingredients of said terminal electrodes to the solder layers.
According to the first aspect of the present invention, since the metal composition of the solder layers is an Snxe2x80x94Sb system, no lead is contained and there is no concern over environmental pollution. Further, since the metal composition of the solder layers is, by ratio of weight percent, Sn/Sb=70/30 to 90/10 and the conductive ingredients of the terminal electrodes of the electronic device body diffuse into the solder layers to form diffusion layers at the soldered parts, it is possible to provide an electronic device with external terminals superior in tensile strength, load resistance, and other mechanical strength properties.
Preferably, said external terminals are comprised of members able to elastically deform in a length direction of said electronic device body. By elastic deformation of the external terminals, it is possible to absorb deformation due to the difference in heat expansion between the electronic device body and the circuit board on which it is mounted.
Preferably, said solder layers are comprised of an Snxe2x80x94Sb high temperature solder having a solidus temperature of at least 240xc2x0 C.
Preferably, said terminal electrodes have layers comprised mainly of Cu and the ingredients diffused into said diffusion layers are comprised mainly of Cu.
Preferably, said terminal electrodes have underlayer electrodes comprised mainly of Cu, Ni plating layers formed on the surfaces of said underlayer electrodes, and Sn plating layers formed on the surfaces of the Ni plating layers, and the Cu diffused into said diffusion layers diffuses there by passing through said Ni plating layers and Sn plating layers.
According to a second aspect of the present invention, there is provided an electronic device with terminal electrodes having terminal electrodes and external terminals of the electronic device body electrically bonded through solder layers, wherein said solder layers and terminal electrodes are formed between them with diffusion layers having thicknesses of at least 5 xcexcm formed by diffusion of conductive ingredients of said terminal electrodes into the solder layers.
In the electronic device with external terminals according to the second aspect of the present invention, since diffusion layers of thicknesses of at least 5 xcexcm are formed at the solder layers bonding the terminal electrodes and external terminals, the tensile strength between the terminal electrodes and external terminals is improved.
Preferably, said solder layers are comprised of an Snxe2x80x94Sb high temperature solder having a solidus temperature of at least 240xc2x0 C.
Preferably, said terminal electrodes have underlayer electrodes comprised mainly of Cu, and the conductive ingredients diffused in said solder layers are Cu.
Preferably, said terminal electrodes have underlayer electrodes comprised mainly of Cu and Ni plating layers formed on the surfaces of the underlayer electrodes, and the Cu of the conductive ingredients diffusing into the solder layers diffuse there through the Ni plating layers.
Preferably, said solder layers are comprised of an Snxe2x80x94Sb high temperature lead-free solder, and the ratio between the Sn and Sb in the solder layers is, by ratio of weight percent, in the range of Sn/Sb=70/30 to 90/10.
Preferably, said external terminals are comprised of members able to elastically deform in a length direction of said electronic device body.
The method of production of an electronic device with external terminals according to the first aspect of the present invention comprises the steps of preparing a high temperature lead-free solder comprised of a cream solder having a metal composition of, by ratio of weight percent, Sn/Sb=70/30 to 90/10 and a solidus temperature of at least 240xc2x0 C., depositing said high temperature lead-free solder between the external terminals and external terminals of the electronic device body, and heating said terminal electrodes and external terminals to a temperature range of a maximum temperature of 310xc2x0 C. to 340xc2x0 C. and bonding them through solder layers, when heating and bonding said terminal electrodes and external terminals, conductive ingredients of said terminal electrodes diffusing into said solder layers and forming diffusion layers between said solder layers and external terminals.
Preferably, the metal composition has a ratio by weight percent of Sn/Sb=approximately 80/20.
The method of production of an electronic device with external terminals according to the second aspect of the present invention comprises the steps of preparing a high temperature lead-free solder comprised of a cream solder having a metal composition of, by ratio of weight percent, Sn/Sb=approximately 90/10 and a solidus temperature of at least 240xc2x0 C., depositing said high temperature lead-free solder between the external terminals and external terminals of the electronic device body, and heating said terminal electrodes and external terminals to a temperature range of a maximum temperature of 325xc2x0 C. to 350xc2x0 C. and bonding them through solder layers, when heating and bonding said terminal electrodes and external terminals, conductive ingredients of said terminal electrodes diffusing into said solder layers and forming diffusion layers between said solder layers and external terminals.
According to the methods of production of the first and second aspects of the present invention, it is possible to efficiently produce electronic devices with external terminals according to the first and second aspects of the present invention.
In the first and second aspects of the present invention, preferably the thicknesses of said diffusion layers are at least 5 xcexcm. Note that the upper limit of the thicknesses of the diffusion layers is not more than the thicknesses of the solder layers. Specifically, the thicknesses of the diffusion layers are preferably 5 to 20 xcexcm, more preferably 5 to 10 xcexcm.